The invention relates to an X-ray diagnostic installation for angiographic X-ray examinations, comprising an image intensifier-television installation, at least one image memory, a subtraction device for the subtraction of image data occurring at various times, and a monitor, in which the current video signal and a video signal representing an averaging of several images in the image memory are supplied to the subtraction device, in which a maximum detector is connected to the subtraction device, which detector recognizes the maximum of the contrast medium flow, and wherein the maximum detector is connected to the image memory for initiating the storage of an image taken with an optimum amount of contrast medium present. Subtraction images find use in the case of fluoroscopic examinations, particularly in the case of selective angiography, in order to make blood vessels clearly visible which, in the normal radiograph, can be poorly recognized or on which bone structures are superimposed.
In the case of the X-ray diagnostic installation of the initially cited type according to the German Pat. No. 3,215,552 corresponding to U.S. Pat. No. 4,543,604, a mask corresponding to a radiograph without contrast agent, which may be termed an empty image, can be read into a second image memory under manual control, or automatically by means of a detector. Subsequently, a video signal averaged over several images can be continuously read into the first image memory. Through subtraction of this stored video signal from the current video signal the maximum of the contrast agent flow is detected by the maximum detector which then terminates the storage so that an image remains permanently stored in the first image memory which corresponds with the presence of a desired density of contrast medium and which may be termed a filling image. In the case of manual triggering to initiate the storage of an empty (non-contrast medium-filled) image pursuant to visual control, conditioned by the reaction time of the observer and by the minimum recognizability of the rise of the contrast medium density, the taking of an empty image radiograph at the optimum time can be achieved only in rare instances. Moreover, with regard to a possible movement of the patient, the radiographic time of the empty image radiograph must lie as closely as possible before the rise of the contrast agent density.
In order to avoid these difficulties a detector for the automatic detection of the empty image is described in the aforementioned German patent in which the average video signal is likewise subtracted from the current video signal. If the difference signal exceeds a threshold value, the detector causes an empty image to remain permanently stored in a second image memory. However, the problem occurring here is that, given a movement of the patient already prior to the injection of the contrast medium or during the injection, due to the injection shock, the detector can respond to this. On the other hand, using the detection means described in the German patent, the maximum of the contrast medium flow, on account of its strong change, can be satisfactorily determined.